Drop redundant `memory_bytes` metrics from ruby_sampler

What does this MR do?

This removes the redundant ruby_memory_bytes metric from RubySampler, which is also tracked as ruby_process_resident_memory_bytes.

The latter has been around since GitLab 12.0, and removing the older metric might be a breaking change if a customer is using it in their dashboards and haven't migrated to the new metric yet.

I have therefore added a changlog entry documenting this as a feature removal.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Closes #216584 (closed)